1. Field of the Invention
The present invention relates to a servo signal inspecting device and a method for inspecting a servo signal.
2. Description of the Related Art
Magnetic tape is widely used as recording medium for reproducibly recording data. In such a magnetic tape, in order to increase recording density per unit area, a number of data bands for recording data are formed along the magnetic tape and packed in a width direction of the magnetic tape. For various reasons, the magnetic tape repeatedly and slightly shifts in the width direction during running, and it may become difficult for a recording/reproducing head to completely trace the data bands during recording/reproducing of data. Therefore, in this type of device for recording/reproducing data onto/from a magnetic tape, during recording/reproducing of data, tracking of data bands of the magnetic tape by the recording/reproducing head is dynamically controlled based on a servo signal which has been written in advance on the magnetic tape. Specifically, a servo signal read head integrally formed with the recording/reproducing head reads the servo signal to detect deviation of the recording/reproducing head in the width direction from the normal position, and based on the detected deviation the recording/reproducing head is moved to cancel the deviation, so that the magnetic tape is properly tracked and running condition of the magnetic tape is controlled. Such a servo signal is written in advance by a specific servo writer in a running direction of the magnetic tape (see, for example, Unexamined Japanese Patent Publication No. 2003-141836 (paragraphs 0002-0005)).
As for the servo signal, timing based servo (hereinbelow, simply referred to as “TBS”) signal is known. As shown in FIG. 2A, this TBS signal is written on servo bands SB1, SB2, SB3, SB4 and SB5 on a magnetic recording surface of the magnetic tape MT, with adjacent two servo bands flanking data bands DB1, DB2, DB3 and DB4, respectively. The servo signals written on the servo bands SB1, SB2, SB3, SB4 and SB5 are retrieved by the servo signal read head that slidably contacts with the magnetic tape MT while the magnetic tape MT is run by a tape running system. For example, as shown in FIG. 2B, one unit of the TBS signal is formed of a nonparallel pair of servo pattern sets each set composed of 5 servo patterns (SP1A, SP1B, SP1C, SP1D and SP1E; and SP2A, SP2B, SP2C, SP2D and SP2E), and another nonparallel pair of servo pattern sets each set composed of 4 servo patterns (SP3A, SP3B, SP3C and SP3D; and SP4A, SP4B, SP4C and SP4D), which unit is written on the servo band SB at a predetermined spacing along a running direction A of the magnetic tape MT. An interval between the servo pattern set SP1A, SP1B, SP1C, SP1D and SP1E and the servo pattern set SP2A, SP2B, SP2C, SP2D and SP2E, and an interval between the servo pattern set SP1A, SP1B, SP1C, SP1D and SP1E and the servo pattern set SP3A, SP3B, SP3C and SP3D are measured by means of a time interval of pulse peaks of the servo signals reproduced by the servo signal read head. In accordance with the results, running condition of the magnetic tape is controlled. For example, as shown in FIG. 2C, time intervals are measured, such as a time interval AB0 between the pulse peak PA1 in the servo pattern SP1A and the pulse peak PB1 in the servo pattern SP2A, and a time interval AC0 between the pulse peak PA1 in the servo pattern SP1A and the pulse peak PC1 in the next servo pattern SP3A. Subsequently, the ratio AB0/AC0 is calculated, and in accordance with the results, running condition of the magnetic tape is controlled.
In the process of manufacturing and inspecting the magnetic tape for servo system utilizing such a TBS signal for tracking control, a servo signal is written on the magnetic tape, and at the same time it is necessary to read the servo signal and to examine the precision of the servo signal, in order to evaluate quality of the written servo signal.
In general, the above-mentioned measurement of the time interval between the pulse peaks is performed by comparing a standard clock and timing of the pulse peaks. Therefore, the precision of the servo signal measuring system in the servo signal inspecting device mainly depends on the precision of the standard clock used for measuring the peak interval. Generally, in a servo signal inspecting device on the magnetic tape for the servo system utilizing TBS signal, the minimum resolution in the measuring system is defined as Lclk=T×v [m], where T [s] is a period of the standard clock for measuring the time interval between peak pulses, v [m/s] is the running speed of the tape in the tape running system, and Lclk is a length of the magnetic tape per unit clock by the standard clock. For example, when the time interval between the peaks is measured at the running speed of 5 m/s and with the standard clock signal of 50 MHz, the minimum resolution per standard clock is obtained as follows: 20 ns×5 m/s=100 nm. In the case of the conventional magnetic tape with the track pitch of approximately 20-30 μm, when the above-mentioned level of resolution is used for measuring the time interval of the peak pulses, quality of the servo signal is inspected with a sufficient precision.
On the other hand, much higher capacity is demanded in the magnetic tape used with recent data recording/reproducing system. To meet this demand, some methods for narrowing the tracks have been proposed. For example, in unpublished Japanese patent application No. 2004-145136, the present inventor previously has proposed a method in which an angle between the two (nearly inverted V-shaped) sets of the servo patterns of the TBS signal is widened; and also in Japanese unexamined patent laid-open application publication No. 2004-318977, the present inventor previously has proposed a method in which S/N ratio of the servo signal is improved by the use of a DC write (writing with direct current magnetization). Another example includes a method in which Mrt of the magnetic tape is reduced. With this change in methods for writing servo signal for the purpose of obtaining narrower tracks and with the improvement in quality of the magnetic tape itself, the demand in quality for servo signal became stricter. Therefore, further improvement in the measurement precision of the servo signal is demanded in a device for inspecting quality of the servo signal.
It is therefore desired to provide a servo signal inspecting device and a method for evaluating quality of the servo signal by measuring the servo signal with a sufficient resolution, even when narrower data bands are made on the magnetic tape.